THE CELL AND PROTOPLASM. 77 



mother liquid. This supposed substance Schwann 

 called the cytoUastema, and he thought it existed 

 between the cells or sometimes within them. 

 For example, the fluid part of the blood is the 

 cytoblastema, the blood corpuscles being the 

 cells. From this structureless fluid the cells 

 were supposed to arise by a process akin to 

 crystallization. To be sure, the cells grow in a 

 manner very different from that of a crystal. A 

 crystal always grows by layers being added upon 

 its outside, while the cells grow by additions 

 within its body. But this was a minor detail, 

 the essential point being that from a structure- 

 less liquid containing proper materials the organ- 

 ized cell separated itself. 



This idea of the cytoblastema was early thrown 

 into suspicion, and almost at the time of the 

 announcement of the cell doctrine certain micro- 

 scopists made the claim that these cells did not 

 come from any structureless medium, but by 

 division from other cells like themselves. This 

 claim, and its demonstration, was of even greater 

 importance than the discovery of the cells. For 

 a number of years, however, the matter was in 

 dispute, evidence being collected which about 

 equally attested each view. It was a Scotchman, 

 Dr Barry, who finally produced evidence which 

 settled the question from the study of the de- 

 veloping egg. 



The essence of his discovery was as follows : 

 The ovum of an animal is a single cell, and when 

 it begins to develop into an embryo it first simply 

 divides into two halves, producing two cells (Fig 

 8, a and b). Each of these in turn divides, giving 



